Selected doc in Image Field

Yes you can but that will involve Submitting the form(like a Upload Button) and again coming back to the same form using activity chaining.

First you need an Upload Button which will be a submit button.After you select the document, you should click the Upload Button which takes you inside the process model and the permanent document id of the document is generated. Then you need to put an XOR gate which should be able to recognize the Upload action and then link the flow to the same Form(this flow should be completely activity chained). This was you can display the selected document in the image field.

Please let me know if the information was useful.

AFAIK, there's no way of doing so without clicking a button, as Sidhant has suggested above, since the document is not saved to the Appian repository until after leaving the form.